1. Field of the Invention
The present invention relates to a graded-index multimode fiber.
2. Description of Related Art
A graded-index multimode fiber (hereinafter referred to as “GI multimode fiber”), which is one type of multimode fiber, is fabricated by doping at least one dopant, e.g., germanium (Ge), in the core thereof such that the doped core has a higher refractive index (sometimes simply referred to as “index”) than the refractive index of pure silica. Such a GI fiber has a refractive index profile in which the refractive index is highest at the center of the core and continuously decreases toward the boundary between the core and the cladding, as the distance from the center of the core increases.
In a GI multimode fiber having such a configuration, since the light propagating in the outer regions can travel faster than the light propagating in the center region, differences in propagation rates among different modes are minimized, thereby reducing the modal dispersion and increasing transmission bandwidth.
Such a GI multimode fiber has a large numerical aperture, and has been widely used in a transmission line of an optical local area network (LAN). In order to meet a need for a faster optical LAN, techniques to control the refractive index profiles of GI multimode fibers have been researched.
However, further improvement in performance of GI multimode fibers seems almost impossible at present, and wavelength division multiplexing (WDM) is required for increasing transmission bandwidth of GI multimode fibers.
In a conventional GI multimode fiber which has a germanium-containing core, the optimum refractive index profile varies greatly depending on the wavelength of the light signal propagating through the fiber. Accordingly, since a fiber having a refractive index profile optimized at a certain wavelength provides a very small transmission bandwidth at other wavelengths, it cannot be used for wavelength division multiplexing (WDM), which has been discussed in literature, for example in R. Olshansky, “Propagation in glass optical waveguides”, Reviews of Modem Physics, Vol. 51, No. 2, pp. 341–367, 1979).
In addition, chromatic dispersion caused by germanium is significant in wavelength regions far from the zero dispersion wavelength, e.g., the 0.85 μm region, making the transmission bandwidth very small.